Normally, the colours we perceive are determined by the
wavelengths of light reflected by objects in the world around us.
But not all surfaces reflect light the same way. Picture an
iridescent butterfly, for example. It might look drab from one
direction, but explode into bright yellows or purples from another.
That's because of microscopic structures that alter the way light
bounces off the butterfly's wings.

At the NanoPhotonics
Centre at the University of Cambridge, scientists are
tinkering with tiny structures like the ones in butterfly wings to
create crazy new materials that manipulate light and change colour
in strange ways.

"A lot of this stuff is not completely mainstream," said Jeremy
Baumberg, who directs the centre. "People think it's a bit
weird."

During a recent visit to Cambridge, I sat down with Baumberg to
talk about some of the projects he and his colleagues -- engineers,
physicists, chemists, materials scientists, and biologists -- are
working on. This gallery shows off a few highlights.

The secrets behind these multicoloured materials lie in the tiny
nanostructures they're made from: spheres, helices, tangled
gyroids, lattices, super-thin membranes, and stacks. "The nice
thing about all these materials is they're a very visual example of
nanotechnology," Baumberg said. "The features and the colour all
come from structure."

Gen Kamita

Multicoloured balloons
Super-thin, transparent nanolayers reflect different colours depending on their thickness and
the interface between them. If those layers are flexible enough,
they can be folded and stacked to create inflatable membranes that
change colour as they inflate. That's how the lab created these
psychedelic looking bubbles. "You blow air through the membrane,
and you get these colour-changing bubbles," Baumberg said.

In addition to groovier party balloons, such technology could
inspire practical products like tires that change to green when
inflated to the right air pressure.

Jan Martens and Laura Brooks

Colour-changing wallpaper
Your white walls are so boring. What if you could flick a switch
and watch them become something less predictable?

Baumberg and his group are working on wall coatings that can
change colour. These coatings would contain transparent
nanostructures arranged in precise matrices. Changing the
structures' separation -- by running an electrical current through
the coating, for example -- would shift the wavelength, and the
colour, they reflect. The group has already figured out how this
could work, and they've made sheets about the size of a postage
stamp. The challenge now is coming up with a good way to make
sheets big enough to liven up your dreary office.

"The thing that's important to me is not making a tiny bit of
material using expensive technology," Baumberg says. Instead, he's
aiming for materials that are low-cost and easy to produce.

Silvia Vignolini

Biomimetic berry-based structures
Dry a bunch of flowers for a few months and they'll lose much of
their colour, fading into dusky, wilted versions of their original
hues. But not Pollia condensata. These brilliant berries
maintain their bluish purple iridescence for decades.
They're the
most intensely coloured biological thing on Earth.

Last year, Baumberg's team, including collaborator Silvia
Vignolini, worked out the
secret to the berries' transfixing, glittery colour. Rather
than incorporating pigments that fade -- like those in the flower
petals -- the berry's surface cells build colour-generating
structures in their membranes. The structures are made from
cellulose nanofibers, laid down in periodic, twisting helices. The
tiny spirals act as reflective cavities and bounce light around,
ultimately amplifying and reflecting the bluish purple hue.

Now the group is
working on replicating these colour-producing structures
using cellulose nanocrystals. "It's not easy to do at the moment,"
Baumberg said. There are some good reasons for trying, though.
Remember those blue M&Ms that are no longer around? Turns out,
most blue food dyes are carcinogenic. Maybe, if the group gets it
right, they'll come up with a nontoxic dye that can be used to
resurrect those long-lost chocolaty treats.

Stefano Salvatore

Holey gold
A 3D gyroid
metamaterial made from gold sounds totally whack. And it
kind of is. The various twists and turns assumed by the metal
affect the way light moves through it.

Baumberg and his team make this weird material by starting with
a polymer that self-assembles into the gyroid lattice. "It will do
that if you cool it slowly enough, and at the right concentration,"
Baumberg said. The lattice, he notes, "is really beautiful." Then,
they add a second polymer that fills in the empty space. Removing
the original, self-assembling polymer leaves a veiny mold that can
be filled in with gold. Once the gold is in, they dissolve away the
mold, leaving behind a 3D metal lattice. "I call it holey gold,"
Baumberg said. "It has completely different properties from gold.
Light worms its way through differently."

These types of structures could be used to enhance the
conversion of light into electricity in solar cells, or to create
metals with completely different properties than the original.

Jasna Rokegem/Qibin Zhao

Polymer opals
Named for their resemblance to an opal's flickering
colours, these
materials change colour when stretched. What initially
looks like a shiny piece of yellow elastic shifts through green to
blue. Patterns printed onto the colour-changing stretchable also
morph as the material's shape shifts.

These materials are made by laying a matrix of tiny spheres onto
a stretchable fabric. Spheres of a certain size, separated by a
certain distance, will reflect light of a certain colour. But as
you yank on the fabric and increase the distance between the
spheres, the wavelength of light they reflect shifts. Suddenly,
instead of orange, you have something green.

It's tempting to imagine where these colour-changing materials
might find a home. Perhaps not surprisingly, Baumberg says the most
interest the team has gotten has been from the fashion industry.
Already, runways in Paris have showcased garments incorporating
these stretchables. Maybe we'll be able to wear a trippy,
colour-changing jumpsuit to Burning Man next year?

Lindsey Ibbotson

Woodpile nanostructures
In photonics, woodpile structures are layered materials stacked in
much the same way as an actual woodpile -- except of course that
they're not made of wood. Also, they're very tiny. And they control
light in weird ways.

"They're very technically difficult structures to make, on a
100-nanometer scale," Baumberg said. Working in this size range
allows the material to trap and reroute light because it creates
regions within the structure where electromagnetic radiation,
including visible light, cannot pass. Baumberg and his team are
working on creating metal-containing woodpile structures, using
gold wires and other flexible materials. Because these structures
can direct how light (and information) moves through a system,
they're useful in communications systems and sensors.

When these mats are relaxed and the gold particles are near one
another, the material looks like gold and acts like a metal. That
is, electrons can hop from particle to particle. But if you stretch
the mat and increase the distance between the gold nanoparticles,
the material morphs. First, its colour changes to red. And second,
it starts behaving like an insulator since electrons have a tougher
time moving from particle to particle.

Baumberg envisions these kinds of nanoparticle mats finding a
place in sensors -- perhaps in the form of disposable films that
could be coupled to a mobile phone. The mats could be made
sensitive to different things by intercalating different molecules
between the gold particles. "For instance, it could measure the
contaminants in the environment, or in something you ate, or
something in our sweat," Baumberg said. When the right molecules
interact with the particle mat, it will swell and turn red.

Fumin Huang

Plasmonic gold lattice
This tube is made from a rolled up, superthin lattice of gold cups. Each of the cups traps and
concentrates light, allowing certain colours to spiral around the
tube. Those colours change as the tube changes shape, and varying
the intensity of light shined on the tubes causes them to roll and
unroll. The tube, and its cups, are able to control and manipulate
light on a nanoscale. Related technologies are used in molecular
diagnostics.

Mathis Kolle and Nick Gibbons

Colour-changing wrapping paper
Thin, clingy films that reflect a myriad of colours can not only
be styled into balloons (see the first slide in this gallery), they
can be used to wrap stuff up. Baumberg and his team do this by
floating the filmy layers on water, to keep them flat and prevent
them from clinging to everything, then rolling them around objects,
like the human hairs in the image above. We know it's highly
improbable that this kind of technology will ever find its way into
hair salons, but if does -- yes, please.